In a helicopter provided with a single main rotor for forward motion and support purposes, it is known that the fuselage tends to rotate in the direction opposite to the direction of rotation of the said main rotor, in reaction to the torque exerted thereby. It is also known that to combat the reaction torque to which the fuselage is subjected, it is common practice to provide an auxiliary anti-torque rotor suitable for generating transverse thrust and rotated from the same driving source as the main rotor. Thus, the auxiliary, anti-torque rotor absorbs a fraction of the power available from said driving source.
As a result, to improve helicopter performance, it is advantageous for the auxiliary anti-torque rotor to be assisted in its function of providing lateral anti-torque stabilization for the fuselage.
For example, as described in U.S. Pat. No. 2, 818,224, it is possible during forwards flight to off-load said anti-torque rotor by using aerodynamic thrust acting on a fin provided at the tail of said helicopter. Such thrust is commonly obtained by giving the fin a curved profile and by setting it at a certain angle relative to the plane of symmetry of the fuselage. However, when defining a stationary fin, the thrust obtained in this way for a helicopter that is flying with no sideslip depends only on the dynamic pressure of the air on the fin, and is therefore not capable of being modulated. Since the anti-torque force to be exerted varies differently as a function of speed, and also as a function of other flight parameters, it results that optimum off-loading of the anti-torque rotor is possible in practice only for a single set of flying circumstances. It is conventional to optimize about level cruising flight, e.g. at the economic cruising speed. The setting of the fin is then selected so that the thrust exerted thereby off-loads the anti-torque rotor completely or largely at the selected cruising speed. As soon as flight conditions (horizontal speed, vertical speed) or the configuration of the aircraft (mass, external loading) change, then the initial adjustment of the fin is no longer optimum and a certain loss of performance appears. In addition, in downwards autorotation flight, thrust from the fin is superfluous (the torque to be countered from the main rotor is substantially zero), and needs to be balanced by negative thrust provided by the anti-torque rotor. This negative thrust represents a loss of performance and significantly reduces the remaining control range available for controlling the helicopter in yaw.
To avoid the drawbacks and the limitations that appear on using such a stationary off-loading fin, it is possible to implement a rudder airfoil that is steerable, as described in U.S. Pat. No. 2,369,652, for example. Such a rudder airfoil is manually controlled and a gyroscopic system is provided for controlling the gases of the main rotor engine so as to control the power delivered by the engine to maintain torque balance and prevent variations in engine power having effects that are too quick to be tracked by the necessary. readjustments of the anti-torque means.
It may be observed that the anti-torque system of the latter patent suffers from a particularly severe drawback in that it subordinates control of the main rotor to anti-torque control. In addition, it is complex since it makes use of a gyroscope system.